Mechanism analysis of laser coloring of Ti alloy by optical simulation

Abstract

The effects of laser energy density on the coloring effect were examined using a nanosecond fiber laser to color the surface of a TC4 Ti alloy. The surface of samples turns yellow, orange, purple, and blue as the single pulse energy density increases. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS) results demonstrated that a mixed film containing TiN and TiO was formed on the surface of samples after laser coloring. The TiN content in the mixed film reduced with increase in single pulse energy density, whereas the TiO content increased. Based on the equivalent medium and optics theory, the SCOUT optical simulation software was used to create a three-layer structure model of ambient/mixed film/Ti alloy substrate. The superposition of reflection and interference in the mixed film was simulated to examine the effect of the mixed film’s composition and thickness on the reflective spectrum of sample. The reflective spectrum of different colors obtained by simulations and tests was extremely consistent, and the simulation composition and thickness of the mixed film were consistent with the tested results, indicating that the laser coloring mechanism is influenced by light interference between mixed films of different composition and thickness formed on the sample surface.